In general, N-alkyl- and N,N-dialkylbenzothiazole-2-sulfenamides are used as vulcanization accelerators for rubber and are known generally as sulfenamide vulcanization accelerators. These sulfenamide vulcanization accelerators are characterized in that they act slowly on a vulcanization reaction. Therefore, they are less likely to cause scorching in the heat history of a rubber processing process and thus are used in many rubber products such as tires and rubber vibration isolators. N-Monoalkyl compounds such as N-cyclohexylbenzothiazole-2-sulfenamide (CBS) and N-t-butylbenzothiazole-2-sulfenamide (BBS) and N,N-dialkyl compounds such as N,N-dicyclohexylbenzothiazole-2-sulfenamide (DCBS) are used as sulfenamide vulcanization accelerators, and the N,N-dialkyl compounds are reported to have a larger slow acting effect on the vulcanization reaction than the N-monoalkyl compounds.
In a rubber product production process, N-nitrosamine produced during the rubber processing process or storage of rubber products is pointed out to be carcinogenic, and, for example, the concentration of carcinogenic N-nitrosamine in environments (such as in rubber processing process or warehouses) is regulated by German Technical Regulation on dangerous Substances TRGS552.
N-Nitrosamines produced from primary amines are easily decomposed due to their instability and thus are not acknowledged as a problem, whereas many of N-nitrosamines derived from secondary amines are pointed out to be carcinogenic. On the other hand, the above-described German Technical Regulation on dangerous Substances TRGS552 explicitly states that ten N-nitrosamines including N-nitrosodicyclohexylamine, N-nitrosodibenzylamine, N-nitroso-N-methyl-t-butylamine, N-nitroso-N-ethyl-t-butylamine, and N-nitroso-N-n-butyl-t-butylamine are not carcinogenic (non-patent document 3).
At the present time, in the Ministry of Economy, Trade and Industry, safety checks of existing chemical substances are successively carried out. Tests on the degree of degradation of chemical substances by microorganisms and the like and tests on the level of bioaccumulation or bioconcentration of chemical substances in the body of fish and shellfish are also carried out as part of the safety checks.
In recent years, as a result of safety checks of existing chemical substances for DCBS which is one of sulfenamide vulcanization accelerators widely used in the production of tires and the like, it was confirmed that DCBS is a hardly degradable substance and is highly bioconcentrative (concentration ratio (BCFSS)=6000 times: Official Bulletin of Economy, Trade and Industry, Dec. 22, 2005). As a result, DCBS was designated as a Type I Monitored Chemical Substance.
Accordingly, the development of sulfenamide vulcanization accelerators that are safe and can act effectively and slowly on vulcanization has been strongly desired. Accordingly, in designing sulfenamide vulcanization accelerators, it is important that an alkyl group be selected while taking into consideration the fact that the slow acting properties for vulcanization are greatly different depending upon the type of the alkyl chain even when secondary amines are used, as well as measures for solving the problem of N-nitrosamine and bioconcentration of sulfenamides.
In view of the above matters, the present inventors have made extensive and intensive studies and, as a result, have found that N-alkyl-N-t-butylbenzothiazole-2-sulfenamides represented by formula [1]:

wherein R represents methyl, ethyl, n-propyl, or n-butyl. can be used as vulcanization accelerators that have slow acting properties for a vulcanization reaction equal or superior to DCBS, do not produce carcinogenic N-nitrosamine, and do not cause environmental problems such as bioconcentration. This has led to the completion of the present invention.
Among these compounds, only N-methyl-N-t-butylbenzothiazole-2-sulfenamide (BMBS) have a registry number (registry No. 139361-76-5) in Chemical Substance Registration System in Chemical Abstracts Service (CAS Registry Number (139361-76-5)). Non-patent document 4 which is a sole document cited from the Chemical Substance Registration System describes that N-methyl-N-t-butylbenzothiazole-2-sulfenamide (BMBS) has the same vulcanization behavior as CBS. In non-patent document 4, however, there is no description on specific experimental facts, for example, synthetic methods for samples used, chemical and spectroscopic properties of the samples, the type and compounding amount of rubbers used for evaluation, experimental results on vulcanization behavior including scorch time, and dynamic property values of the resultant vulcanized rubbers, and the basis on which the authors of non-patent document 4 conclude that N-methyl-N-t-butylbenzothiazole-2-sulfenamide (BMBS) has the same vulcanization behavior as CBS is unknown at all.
N-Ethyl-N-t-butylbenzothiazole-2-sulfenamide (BEBS), N-n-propyl-N-t-butylbenzothiazole-2-sulfenamide (BPBS), and N-n-butyl-N-t-butylbenzothiazole-2-sulfenamide (BBBS) are not, of course, reported to have been synthesized and do not have a registry number in Chemical Substance Registry System in Chemical Abstracts Service. Further, it is a matter of course that there is no report about the use of these compounds as vulcanization accelerators. Sulfenamides containing a branched alkyl group on nitrogen, for example, N-(2-alkyl)-N-t-butylbenzothiazole-2-sulfenamides, are described in patent document 1. These sulfenamides are characterized in that they per ser are solid at ordinary temperature. Due to this characteristic feature, the alkyl group on located nitrogen is limited to one tertiary alkyl and one secondary alkyl or cycloalkyl, and patent document 1 does not refer to sulfenamides according to the present invention in which the alkyl groups on located nitrogen are one t-butyl group and one primary alkyl group. Further, patent document 1 has no description on safety and hygiene, for example, carcinogenesis of N-nitrosamine produced from the corresponding sulfenamide and the accumulation or concentration of sulfenamide.
Further, regarding synthesis methods for sulfenamides, sulfenamides are generally synthesized by adding chlorine or sodium hypochlorite into a mixture of a salt of 2-mercaptobenzothiazole or bis(benzothiazol-2-yl)disulfide with a corresponding amine. This synthetic method, however, results in the inclusion of many impurities in a product. The present inventor has also made extensive and intensive studies on this matter and, as a result, has found that high-purity sulfenamides can be produced by a production process comprising previously reacting amine with chlorine or sodium hypochloride to give N-chloroamine and then reacting N-chloroamine with bis(benzothiazol-2-yl)disulfide in the presence of a base in an organic solvent.    Non-patent document 1: Chikosei Karyuyo Yakuzai Toshiteno S—N Kagobutsu (S—N compounds as slow acting vulcanization agent) (part 1), NIPPON GOMU KYOKAISHI, Vol. 51, No. 11, 1978, p. 842-852    Non-patent document 2: Chikosei Karyuyo Yakuzai Toshiteno S—N Kagobutsu (S—N compounds as slow acting vulcanization agent) (part 3), NIPPON GOMU KYOKAISHI, Vol. 52, No. 1, 1979, p. 34-40    Non-patent document 3: Technische Regeln fur Gefahrstoffe TRGS 552 N-Nitrosoamine (2007)    Non-patent document 4: IARC Scientific Publications, 105, 592 (1991)    Patent document 1: Japanese Patent Publication No. 11214/1973